1. Field of the Invention
This invention relates to apparatus and techniques for improving the accuracy of radiometric measurements of properties of solid-liquid suspensions. In particular, this invention relates to improvement in the accuracy of measurement of gamma- or x-ray transmission through, or scatter from, suspensions (slurries) of minerals (including coal) in water, where the objective of such measurements is to determine slurry density or elemental composition, for the purpose of controlling industrial mineral processing operations.
Data from which slurry density or slurry elemental composition may be reduced can be obtained by measuring the intensity of gamma- or x-electromagnetic radiation transmitted through, scattered from or excited from a mineral slurry. Under circumstances where the slurry is conveyed in equipment containing air or other gases, measurement accuracy can be impaired by the entrainment of variable quantities of bubbles of these gases in the slurry, where these variable quantities of gas are present at the point where the radiation measurement is made. The extent to which measurement accuracy is impaired is proportional to the volume fraction of gas in the slurry.
Inaccuracies of measurement due to the presence of gases, usually air, in slurries are usually minimised as far as possible by conveying the slurry through a series of units of equipment, such as low-head pump-sump units, so that any air present tends to be present at a constant volumetric concentration. (By "low-head", it is meant that the pressure-head of the pump is only sufficient to move the slurry and the pressure may be in the region of 15 pounds per square inch (p.s.i.) (103.4 kPa) absolute.)
However, the percentage of air in the slurry is still significant and may for example be in the range of 1%-5% by volume.
2. Description of the Prior Art.
One solution which has been proposed to remove the gas is a method of vacuum de-gassing where the slurry sample is pumped into a vacuum chamber, most of the gas comes out of the slurry, and the nearly gas-free slurry is then passed through a measuring zone. The solution has a number of problems, particularly in practical application in an industrial plant. Vacuum techniques are difficult for the operator with the need to keep the evacuated space effectively sealed and the difficulty of removing the slurry therefrom. This method is only suitable for very small sample flow and very fine materials.
It is extremely difficult to accurately measure the amount of gas in a slurry. In the laboratory, a gas-free calibration slurry is prepared and readings are taken as measured quantities of gas are introduced into the slurry. By comparing the radiation counts at the various gas levels with the counts from the slurry under consideration, the volume of gas in the slurry can be determined. The accuracy of this determination depends on the degree in which the calibration slurry is initially gas free. In addition, due to gravitational effects, the gas added to the calibration slurry forms bubbles which rise out of the slurry, affecting the results.